Method of making semiconductor integrated circuit, pattern detecting method, and system for semiconductor alignment and reduced stepping exposure for use in same

ABSTRACT

According to the present invention, in making alignment between a semiconductor integrated circuit wafer and a mask or a reticle in light exposure of the wafer with a monochromatic light such as g-, i- or h- line of a mercury lamp, using a reduced stepping exposure system, light from a predetermined pattern on the wafer is taken out to an off-axis position and observed according to a through-the-lens method; in this case as a characteristic feature of the invention, the observation light is taken out from below the reticle and is passed through chromatic aberration correcting lenses, thereby permitting the use of a polychromatic or continuous spectrum light.

BACKGROUND OF THE INVENTION

The present invention relates to an alignment technique and moreparticularly to a technique which is effective in the application toalignment of a semiconductor wafer (hereinafter referred to simply as"wafer") with respect to a mask in a reduced projection exposure step inthe production of a semiconductor device.

For transferring a circuit pattern formed using a light shielding filmon a mask such as a reticle onto a wafer, there usually is adopted amethod which employs a reduced projection exposure system. In this case,the wafer-mask alignment is performed in the following manner forexample.

Illumination light is radiated through a reduced projection lens to analignment mark formed in a stepped uneven shape on the surface of awafer, then reflected light from the alignment mark is made incident ona TV camera through a beam splitter, etc., and an electric signal isdetected on the basis of the quantity of light of the reflected light tothereby grasp the position of the alignment mark and effect positioningof an area to be exposed of the wafer relative to the mask. Such apositioning technique is generally called "through the lens" (TTL)method.

As a literature pointing out problems to be solved of the TTL methodthere is "Nikkei Micro Devices," Nikkei McGraw-Hill Inc. (Dec. 1, 1987),pp.70-72.

An alignment technique commonly adopted according to the TTL method willbe explained below with reference to FIG. 5.

In FIG. 5, the numeral 71 denotes a wafer as an object to be exposed;numeral 72 denotes a reduced projection lens for exposure which ispositioned just above the wafer 71; numeral 73 denotes a reticle as amaster plate; numeral 74 denotes a TV camera as a recognizer; andnumeral 75 denotes a mercury lamp serving as both an exposure lightsource and an illumination light source. On the optical path of thereduced projection lens 72 and TV camera 74 there are disposed areflecting mirror 76, a relay lens 77 and a beam splitter 78. Lighttransmitted by the beam splitter 78 is received by the TV camera. On theother hand, between the mercury lamp 75 and the beam splitter 78 aredisposed a band pass filter 80 which permits only E-line (546 nm) topass therethrough out of rays provided from the mercury lamp 75, and acondenser lens 81.

Thus, E ray, which is a monochromatic light ray, is used as anillumination light for the detection of a pattern, while at the time ofexposure there is used G-line (436 nm) which is an exposure light.

The illumination light is radiated by the beam splitter 78 onto thewafer 71 via the relay lens 77, reflecting mirror 76 and reducedprojection lens 72, and the reflected light travels backward through theabove path and is incident on the TV camera 74. On the basis of theimage recognized by the TV camera 74 there is made detection of waveformand a central position of such an alignment mark 6 on the wafer as shownin FIG. 4(a) is calculated.

Further, in FIG. 17(a), numeral 201 denotes a wafer as an object to beexposed; numeral 202 denotes a reduced projection lens for exposuredisposed just above the wafer 201; numeral 203 denotes a reticle as amaster plate; numeral 204 denotes a TV camera as a recognizer; andnumeral 205 denotes a mercury lamp as an exposure light source. On theoptical path of the reduced projection lens 202 and TV camera 204 areare disposed a reflecting mirror 206, a relay lens 207 and a half mirror208. Reflected light transmitted through the half mirror 208 is receivedby the TV camera 204. On the other hand, between the mercury lamp 205and the half mirror 208 are disposed a band pass filter 210 whichpermits only E-line (546 nm) to pass therethrough out of the raysprovided from the mercury lamp 205, and a condenser lens 211.

Illumination light emitted from the mercury lamp 205 is applied onto thewafer 201 via the half mirror 208, relay lens 207, reflecting mirror 206and reduced projection lens 202, while the reflected light from thewafer 201 travels backward along the above optical path and is receivedby the TV camera 204, and on the basis of the image recognized by the TVcamera 204 there is made waveform detection.

FIG. 17(b) is a partially sectional view schematically showing analignment pattern 212 formed on the wafer 201.

The alignment pattern 212, which is formed in synchronism with a wiringpattern, etc. on the wafer 201, has a concave section with both innerside-ends formed edge-like. Illumination light is radiated verticallyfrom above to the wafer surface formed with such alignment pattern 212,and it has been premised that the light reflected by the wafer surfacewill travel backward along the same optical path as that of theillumination light.

As shown in the same figure, when the alignment pattern 212 is formed onthe wafer 201 in an ideal condition without distortion, the resultingwaveform exhibits peak values at the paired edge portions as shown inFIG. 17(c). A central position [(X_(R) +X_(L))/2] calculation of thealignment mark 212 is determined by calculation on the basis ofcoordinates X_(R), X_(L) of both peak values, and alignment is effectedusing it as a reference value X_(O).

Further, a brief explanation will be made below about prior artpublications which may be related to the present invention.

In Suzuki et al.'s Japanese Patent Laid-Open No. 293718/87 (laid openDec. 21, 1987) there is disclosed a correction lens system for thecorrection of spherical aberration, astigmatism and coma aberration in astepper having aligning optical system using a single wave length light.

In Komoriya et al.'s Japanese Patent Laid-Open No. 177625/85 (laid openSept. 11, 1985) there is shown an example of inserting a chromaticaberration correcting lens system in an off-axis portion of an on-axisaligning optical system in a stepper having an aligning optical systemusing a continuous spectrum.

In Sugiyama's Japanese Patent Laid-Open No. 203640/86 (laid open Sept.9, 1986) there is disclosed a structure for conducting light which haspassed through a reticle or a mask to a detection system through achromatic aberration correcting lens in an aligning optical system of astepper similar to the above.

Further, in Shiba et. al.'s Japanese Patent Laid-Open No. 251858/86(laid open Nov. 8, 1986) it is disclosed that a cylindricalinner-surface mirror is used in the feed path of an exposure lightsource.

BRIEF SUMMARY OF THE INVENTION

However, the present inventors found that the following problems wereinvolved in the reduced projection exposure system of the structureshown in FIG. 5.

The wafer as an object to be exposed is fed to the reduced projectionexposure system having a photoresist film 82 applied thereon. Theapplication of the photoresist film 82 is performed by dropping liquidresist under rotation of the wafer 71 and utilizing the spread of theresist liquid caused by centrifugal force.

Consequently, due to the centrifugal force during rotation mentionedabove, the photoresist film 82 becomes non-uniform in its thickness nearthe stepped portions of the alignment mark; particularly, the depositedshape of the photoresist film 82 becomes asymmetric with respect to thecenter of the stepped pattern. In this connection, as the illuminationlight for the alignment there usually is employed G-line (436 nm) at asingle wave length (monochromatic light) which ray is used for exposure,resulting in that an interference fringe formed by the reflected lightfrom the upper surface of the stepped pattern and that from the uppersurface of the photoresist film 82 becomes asymmetric, and a signalvoltage waveform obtained from the said interference fringe which is animage recognized by the TV camera also becomes asymmetric andcomplicated. Consequently, it sometimes becomes difficult to detect thestepped pattern of the alignment mark.

In this connection, several means are introduced in the foregoingliteratures for solving the above-mentioned problem, but none of themare satisfactory.

Also in FIGS. 17(a) to 17(c), during formation of the alignment pattern212 of aluminum, there arises unevenness in film thickness at the bottomof the stepped portion so the same pattern becomes asymmetric insectional shape, resulting in that the reflection angle of light isdisturbed, causing distortion in the waveform of edges detected, and soit is difficult to grasp the coordinates of edge positions exactly.

Further, in the event of breakage of an edge at an end portion thereof,it sometimes becomes difficult to grasp the coordinates of the edgeposition due to irregular reflection from the broken surface.

The present invention has been accomplished taking note of the problemsmentioned above. It is an object thereof to provide a technique capableof accurately effecting the detection of a pattern on a wafer coatedwith a photoresist film and thereby capable of realizing a high accuracyalignment.

It is another object of the present invention to provide a techniquecapable of detecting edge coordinates accurately and enhancing thealignment accuracy without being influenced by the distortion of analignment pattern.

It is a further object of the present invention to permit stepperalignment using a polychromatic light or "white" light.

It is a still further object of the present invention to provide areference optical system of a stepper capable of removing variousaberrations accurately.

It is another object of the present invention to provide a steppercapable of easily effecting a sequential control such as reducedmagnification.

It is yet another object of the present invention to provide a positiondetecting method adapted to a double-layer aluminum wiring process.

It is a further object of the present invention to provide an exposureprocess adapted to a wafer process for a memory IC such as DRAM having adouble-layer aluminum wiring (incl. multi-layer Al wiring).

It is a still further object of the present invention to provide amethod of improving the Koler's illumination difficult to induceinterference.

Typical embodiments of the invention disclosed herein will be summarizedbelow.

An embodiment of the invention is provided with a light source forradiating a continuous spectrum light of at least two wave lengths to anobject to be exposed through a reduced projection lens, a recognizerwhich receives the light reflected from the object to be exposed andrecognizes position, and a chromatic aberration correcting lensmechanism disposed on the optical path of the reflected light in aposition between the object to be exposed and the recognizer.

According to the above means wherein the chromatic aberration correctinglens mechanism is disposed on the illumination light optical path, it ispossible to use a continuous spectrum light of two or more wave lengthsas the pattern illumination light by adjusting the focal length inaccordance with each wave length. Consequently, for example, even wherethe photoresist film is non-uniform in thickness and asymmetric withrespect to the alignment pattern of the detection area, it is possibleto prevent the impossibility of detection caused by an asymmetricinterference fringe such as that in the use of a single wavelengthlight, and thereby enhance the positioning accuracy at the time ofalignment.

According to another typical embodiment of the invention, in opticallydetecting a stepped pattern formed on a semiconductor device, anillumination light is radiated t the stepped portion inclinedly relativeto a vertical reference optical axis through the semiconductor devicesurface.

Further, a first coordinate value obtained by radiating an illuminationlight to the stepped portion inclinedly relative to the verticalreference optical axis through the semiconductor device surface iscompared with a second coordinate value obtained by the radiation of avertical illumination light parallel to the vertical reference opticalaxis to thereby calculate the amount of correction for the secondcoordinate value of the vertical illumination light. And in the patterndetection which follows, the said amount of detection is added to thecoordinate value obtained by the radiation of the vertical illuminationlight to calculate a true coordinate value.

According to the above means, the illumination light is radiated at apredetermined angle of inclination relative to the vertical referenceoptical axis of the pattern, whereby it is possible to accurately detectthe position of one of both edges of the pattern. Thus, by repeating theradiation of the illumination light to the pattern under varyinginclination angles, it becomes possible to accurately detect theposition of even a distorted or sectionally asymmetric pattern, therebypermitting enhancement of the alignment accuracy.

Further, by calculating the amount of correction through the comparisonbetween the first and the second coordinate value, the amount ofdeviation in the detection using the vertical illumination light becomesclear, so an efficient alignment processing can be realized in thesucceeding detection of position by correcting the data obtained fromonly the vertical illumination light.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a perspective view of principal components of a reducedprojection exposure system according to embodiment-1 of the presentinvention;

FIG. 1(b) is a system diagram showing an optical system used in thedetection of a pattern according to the embodiment-1;

FIGS. 2(a) and 2(b) are explanatory views each showing a chromaticaberration correcting lens used in the embodiment-1;

FIG. 2(c) is a perspective view showing an astigmatism correct lens usedin the embodiment-1;

FIGS. 3(a) to 3(c) explanatory views conceptually showing correctionprinciples using the chromatic aberration correcting lens in theembodiment-1;

FIG. 4(a) and 4(b) are explanatory views showing the relation between analignment mark formed on a wafer and a detected wave form in theembodiment-1;

FIG. 5 is a system diagram showing an optical system used in theconventional pattern detection;

FIGS. 6(a) and 6(b) are views for explaining chromatic aberration;

FIG. 7 is a perspective view of principal components of a reducedprojection exposure system used in embodiment-2(I) of the presentinvention;

FIG. 8 is a system diagram for explaining an optical system usedtherein;

FIGS. 9(a) and 9(d) are explanatory views each showing an inclined stateof an optical axis inclined by an optical axis moving portion;

FIGS. 10(a) and 10(b) are views explanatory of chromatic aberration inthe embodiment-2(I);

FIGS. 11(a) and 11(b) are a partially sectional view and a plan view,respectively, showing an alignment pattern formed on a wafer;

FIGS. 12(a) to 12(f), 13(a) to 13(f) and FIGS. 14(a) to 14(f) areexplanatory views showing pattern detecting methods used in theembodiment-2(I);

FIGS. 15(a) and 15(b) are flow charts each showing a signal processingprocedure in the pattern detection of the embodiment-2(I);

FIG. 16 is a partial system diagram showing the arrangement ofillumination sources used in a reduced projection exposure systemaccording to embodiment-2(II) of the present invention;

FIG. 17(a) is a system diagram for explaining the alignment techniqueaccording to the TTL method used in the prior art;

FIGS. 17(b) and 17(c) are a partially sectional view for explaining aformed state of an alignment pattern in the prior art and a view of theresulting signal waveform, respectively;

FIG. 18 is a perspective view showing principal components of a reducedprojection exposure system having an alignment device according toembodiment-3 of the present invention;

FIG. 19 is an explanatory view showing an illumination light sourceportion used therein;

FIG. 20 is an explanatory view showing a modified illumination lightsource;

FIG. 21 is a view explanatory of the principle of non-Koler'sillumination;

FIG. 22 is a view showing a virtual image in the position of entrancepupil of a reduced projection lens;

FIG. 23 is a view explanatory of a correlation between the shape of analignment mark and detection signals;

FIG. 24 is a diagram showing changes in interference intensity in aphotoresist film in Koler's illumination and critical illumination;

FIG. 25 is a principle diagram of Koler's illumination;

FIG. 26 is a principle diagram of critical illumination;

FIG. 27 is a schematic wave length - light intensity distributiondiagram showing a wave length construction of a reference light used inchip alignment in the present invention;

FIG. 28 is a layout plan view of a pair of optical barrels and aprojection exposure lens system for off-axis global alignment used inpre-alignment in the invention;

FIG. 29 is a schematic light ray tracing diagram showing a path of amain light ray in an exposed state according to the present invention;

FIGS. 30 to 34 are sectional flow charts showing part of themanufacturing process flow in a semiconductor integrated circuit deviceembodying the invention;

FIG. 35 is a top view of a wafer, showing positions of a pair ofalignment patterns for global 10 alignment in the invention;

FIG. 36 is a schematic wafer top view showing positions of mainalignment patterns for chip alignment in the invention;

FIG. 37 is a wafer top view showing scribing lines on the wafer in theinvention;

FIG. 38 is a wafer top view showing the shape and planar layout ofalignment marks for use in both the global and the chip alignment in theinvention;

FIG. 39 is a sectional view in Y direction showing an entire layout of achromatic aberration and astigmatism correcting system (aligning opticalsystem) in the invention;

FIGS. 40 and 41 are sectional view in X and Y directions, respectively,of the said correction lens system;

FIGS. 42 and 43 are sectional views showing detailed structure oflaminated achromatic lenses (types A and C, respectively), the sectionsbeing optional sections including the optical axis because of sphericallenses; and

FIGS. 44 to 46 are schematic sectional views showing fine adjustmentsfor chromatic aberration and astigmatism.

DETAILED DESCRIPTION OF THE INVENTION

Improved points of the present invention will be described hereinunderin terms of embodiments, but it is to be understood that the componentsof the embodiments are replaceable with one another and that theembodiments are part of an integral invention, having something to dowith one another.

Therefore, it is to be understood that the components having referencenumerals whose latter two figures in the drawings are the same fulfilthe same or similar functions in principle unless otherwise described.

(1) Embodiment-1

FIG. 1(a) is a perspective view of principal components of a reducedprojection exposure system according to embodiment-1 of the presentinvention; FIG. 1(b) is a system diagram showing an optical system usedin this embodiment; FIGS. 2(a) and 2(b) are explanatory views eachshowing a chromatic aberration correcting lens; FIG. 2(c) is aperspective view showing an astigmatism correction lens used in thisembodiment; FIGS. 3(a) to 3(c) are explanatory views conceptuallyshowing principles of correction using the chromatic aberrationcorrecting lens in this embodiment; and FIGS. 4(a) and 4(b) areexplanatory views showing the relation between an alignment mark formedon a wafer and a detected wave form.

The reduced projection exposure system of this embodiment has an opticalexposure system which comprises an exposure light source 1 comprising amercury lamp, a condenser lens 2 for condensing an exposure lightemitted from the exposure light source 1, and a reduced projection lens3.

Between the condenser lens 2 and the reduced projection lens 3 isremovably disposed a reticle (master plate) 4 which comprises atransparent quartz glass substrate or the like and an integrated circuitpattern formed thereon using a light shielding film of chromium (Cr) forexample.

On the other hand, under the reduced projection lens 3 is placed a wafer(an object to be exposed) 5 which is movable in a horizontal plane on anXY stage. On the wafer 5 is formed a predetermined alignment mark 6 in astepped fashion as shown in FIG. 4(a), the upper surface of which iscoated with a photoresist film 7 by a rotational application technique.More specifically, in FIG. 1(a), an exposure light emitted from theexposure light source 1 and passed through the reticle 4 is reduced to apredetermined magnification (e.g. 1/5) by the reduced projection lens 3and is then projected onto the wafer 5, whereby the photoresist film 7formed on the surface of the wafer 5 is exposed to a predeterminedpattern.

Near the optical exposure system is disposed an illumination lightsource 8. An illumination light emitted from the illumination lightsource 8 is incident on a beam splitter 12 of a half mirror structurethrough a band pass filter 10 and a condenser lens 11 which are disposedon an optical path of the illumination light.

In this embodiment, the illumination light source 8 comprises an opticalfiber 13 or any other suitable light conducting means for conductinglight from the exposure light source 1, and a cylindrical mirror 14mounted on an end portion of the optical fiber.

The band pass filter 10 permits only E-line (546 nm) and D-line (589 nm)to pass therethrough out of light wave lengths provided from the mercurylamp as the exposure light source 1. The illumination light which haspassed through the band pass filter 10 is incident on the beam splitter12 as a continuous spectrum light in a visible light range. On theoptical axis of the beam splitter 12 are disposed a relay lens 15, achromatic aberration correcting lens 16 and an astigmatism correctionlens 17, thereby permitting the light to reach a reflecting mirror 18which is disposed sideways above the reduced projection lens 3.

On the other hand, a TV camera 20 serving as a recognizer through therelay lens 15 is disposed on the optical axis of the beam splitter 12 ina symmetric position with respect to the relay lens 15.

The chromatic aberration correcting lens 16 which is a characteristiccomponent in this embodiment will be described below in detail.

Prior to the description of the lens 16, chromatic aberration will nowbe explained with reference to FIGS. 6(a) and 6(b). In these figures,the reference marks a and b represent distances up to imaging positionsfrom a lens 21. If the focal length of the lens 21 is f, then, a, b andf are in the following relationship: ##EQU1## When the focal lengthchanges by Δf, the change Δb of the imaging position is as followsaccording to the above equation (1): ##EQU2## On the other hand, asshown in FIG. 8(b), the focal length, f, and refractive index, n, of thelens 21 are in the following relationship: ##EQU3## In the aboveequation (3), R represents a radial length of the spherical surface ofthe lens 21.

From the equation (3), the change Δf in focal length caused by a changeΔn in refractive index is: ##EQU4## Substituting this into equation (2)gives: ##EQU5## Since the imaging magnification, m, is b/a, the aboveequation (5) may be written as: ##EQU6##

From the equation (6) it can be seen that the imaging position changesby Δb with the change Δn of the refractive index. In this connection,since the wave length of light and the refractive index, n, areinversely proportional to each other, the imaging position shifts by Δbtowards the lens 21 as the wave length becomes longer. This is chromaticaberration. The reduced projection lens 3 used in the reduced projectionexposure system is generally designed to have optical characteristicsmost suitable to G-line as the exposure light, so consideration is notgiven to the deviation of focal length in the case of using E- or D-lineas the illumination light.

Where a continuous spectrum light of E- and D-lines is used forpreventing the impossibility of detection caused by interference fringe,the E-line of a short wave length will be focused relatively near thelens, while the D-line of a longer wave length will be focused in aposition farther from the lens. According to our calculation, thedifference in imaging position in the TV camera 20 between both lineswhich have passed through the optical system for alignment is as largeas about several ten millimeters. This point is overcome by thechromatic aberration correcting lens 16 in this embodiment.

More specifically, the chromatic aberration correcting lens 16 has thefunction of maintaining the imaging position constant irrespective ofwhether the incident wave length is large or small. It is adjusted sothat the imaging distance is made small with respect to light of a smallrefractive index and made large with respect to light of a largerefractive index.

The chromatic aberration correcting lens 16, for example as shown inFIGS. 2(a) and 2(b), is constituted by a combination of a convex lens16a comprising a flint glass and a convex lens 16b comprising a crownglass. In this embodiment there are used a pair of chromatic aberrationcorrecting lenses 16 of the construction described above. There may beadopted such a combination as shown in FIG. 2(b). The chromaticaberration correcting lens 16 is not specially limited if only it cancorrect chromatic aberration in a wave length region including E- andD-lines in a chromatic aberration correctable range of λ=500 nm to 590nm in terms of wave length, λ. As to the said chromatic aberrationcorrectable range, adjustment can be made by changing the spacingbetween the paired correction lenses 16.

It is FIGS. 3(a) to 3(c) that conceptually show principles of thechromatic aberration correcting lens 16 described above. FIG. 3(a) showsan imaging distance fe in the incidence of a monochromatic light ofE-line before the correction of chromatic aberration; FIG. 3(b) shows animaging distance fd of D-line, and FIG. 3(c) shows an imaging distancefs of E- and D-lines in the case where the correction of chromaticaberration is made through the chromatic aberration correcting lens 16disposed on the optical path of the illumination light. The imagingdistance fs in FIG. 3(c) is adjusted so as to be an intermediate pointbetween the imaging distances in FIGS. 3(a) and 3(b), that is,approximately fs=(fe+fd)/2. Therefore, where a continuous spectrum lightof E- and D-lines is used as the illumination light, it is possible tomaintain the imaging position constant by suppressing chromaticaberration. As a result, it is possible to prevent the impossibility ofthe detection of position caused by an asymmetric interference fringewhich occurs in the use of a chromatic light of E- or D-line alone, andso a highly accurate alignment pattern detection can be effected by theTV camera 20.

In this embodiment, such an astigmatism correction lens 17 as shown inFIG. 2(c) is disposed on the optical path of the illumination light in aposition between a pair of chromatic aberration correcting lenses 16.The astigmatism correction lens 17 is for correcting the deviation in XYdirection of a detected image on the wafer 5 caused by astigmatic lightbeam, that is, astigmatism. It comprises a combination of a convex lens17a of a cylindrical lens and a convex lens 17b. Thus, according to thisembodiment, an illumination light which has been corrected in bothchromatic aberration and astigmatism is incident on the TV camera 20, soit becomes possible to effect the detection of position through a highlyaccurate image recognition.

The following is an explanation about the method of making alignmentaccording to this embodiment.

First, when an illumination light is radiated from the cylindricalmirror 14 at the end of the optical fiber 13 as the illumination lightsource 8 by moving the XY stage, it passes through the band pass filter10 and the condenser lens 11, then is refracted by the beam splitter 12and illuminates a predetermined area of the wafer 5 via the relay lens15, chromatic aberration correcting lenses 16 and astigmatism correctionlens 17, further through the reflecting mirror 18 and the reducedprojection lens 3. The reflected light from the wafer 5 travels backwardthrough the above path and reaches the beam splitter 12 via the reducedprojection lens 3, reflecting mirror 18, chromatic aberration correctinglenses 16 and astigmatism correction lens 17. Then, the reflected lightpasses through the beam splitter 12 and reaches the TV camera 20 throughthe relay lens 15. To the TV camera 20 is connected a signal processingsection to detect a signal wave form from an image recognized by thesame camera.

It is FIG. 4(b) that shows such detected signal waveform. According tothis embodiment, since the chromatic aberration correcting lenses 16 aredisposed on the optical path of the illumination light, chromaticaberration, i.e., deviation of the imaging position, can be correctedeven where a continuous spectrum light of E- and D-lines is used as theillumination light. As a result, it is possible to avoid the difficultyin detecting the alignment mark 6 caused by an interference wave formwhich is attributable to unevenness in the photoresist film thickness inthe use of a chromatic light as the illumination light source 8, and itis possible to surely grasp detected signals corresponding to thestepped portions of the alignment mark 6 as shown in FIG. 4(b), thuspermitting an exact detection of the position of the same mark. The areato be exposed of the wafer 5 is accurately positioned on the opticalexposure system on the basis of the thus-detected position of thealignment mark 6. Thereafter, the integrated circuit pattern on thereticle 4 is transferred onto the photoresist film 7 of the wafer 5 byan exposure light which has been emitted from the exposure light source1 and passed through the condenser lens 2, reticle 4 and reducedprojection lens 3.

Although in the above description the mercury lamp which is the exposurelight source also serves as the illumination light source, there may beused a separate xenon lamp as the illumination light source. In thiscase, by using a xenon lamp which has a relatively uniform light energyat various wave lengths, it becomes possible to adopt a continuousspectrum light selectively, so by adjusting the correction rate of thechromatic aberration correcting lens 16 and setting an optimumcorrection value, it is possible to prevent the impossibility of thepattern detection caused by the interference of reflected light, thuspermitting a highly accurate detection of position.

Although an embodiment of the present invention has been describedconcretely, it goes without saying that the invention is not limitedthereto and that various modifications can be made within the scope notdeparting from the gist of the invention.

For example, although the cylindrical mirror mounted on an end portionof the optical fiber was shown as an example of the illumination lightsource, there may be adopted any other shape, e.g. prismatic orpyramidal shape.

Further, although in the above description the present invention wasapplied to the alignment technique in the so-called reduced projectionexposure for a wafer as a utilization field of the invention, thisconstitutes no limitation and the invention is widely applicable to thealignment technique in the general reduced projection exposure.

Typical effects attained by the embodiment described above will bedescribed below briefly.

According to the present invention, the chromatic aberration correctinglens mechanism is disposed on the optical path of illumination light toadjust the focal length in response to various wave lengths, whereby itbecomes possible to use a continuous spectrum light of two or more wavelengths as the pattern illumination light. Consequently, for example,even when the photoresist film thickness is non-uniform and asymmetricfor the alignment pattern on the area to be detected, it is possible toprevent the detection of position from becoming infeasible due tointerference as in the use of a single wavelength light, and so thepositioning accuracy at the time of alignment can be enhanced.

(2) Embodiment-2(I)

FIG. 7 is a perspective view of principal components of a reducedprojection exposure system used in embodiment-2(I) of the presentinvention; FIG. 8 is a system diagram for explaining an optical systemused therein; FIGS. 9(a) and 9(b) are explanatory views each showing aninclined state of an optical axis inclined by an optical axis movingportion; FIGS. 10(a) and 10(b) are views explanatory of chromaticaberration in this embodiment; FIGS. 11(a) and 11(b) are a partiallysectional view and a plan view, respectively, showing an alignmentpattern formed on a wafer; FIGS. 12 to 14 are explanatory views showingpattern detecting methods used in this embodiment; FIGS. 15(a) and 15(b)are flow charts each showing a signal processing procedure in thepattern detection of this embodiment.

In FIG. 7, a reduced projection exposure system 101 has an opticalexposure system which comprises an exposure light source 102 comprisinga mercury lamp, a condenser lens 103 for condensing an exposure lightemitted from the exposure light source 102, and a reduced projectionlens 104.

Between the condenser lens 103 and the reduced projection lens 104 isremovably disposed a reticle 105 (master plate) which comprises atransparent quartz glass substrate and an integrated circuit patternformed thereon using a light shielding film of chromium (Cr) forexample.

On the other hand, under the reduced projection lens 104 is placed awafter 107 (an object to be exposed) capable of being adjusted itsposition in the horizontal direction on the upper surface of an XY stage106. On the wafer 107 is formed a predetermined alignment pattern 108 ina stepped fashion as shown in FIG. 11, the upper surface of which iscoated with a photoresist film 110 by a rotational applicationtechnique. More specifically, in FIG. 7, an exposure light emitted fromthe exposure light source 102 and passed through the reticle 105 isreduced to a predetermined magnification (e.g. 1/5) by the reducedprojection lens 104 and is then projected onto the wafer 107, wherebythe integrated circuit pattern on the reticle 105 is transferred ontothe photoresist film 110. As a result, the exposure light-radiatedportion of the photoresist film 110 undergoes a chemical change to forma resist pattern by the same film.

In the vicinity of the above optical exposure system is disposed anillumination light source 111. An illumination light emitted from theillumination light source 111 passes through a condenser lens 112 and anoptical axis moving portion 113 and is reflected at a predeterminedangle by a half mirror 114, then reaches the reduced projection lens 104via a relay lens 115, a chromatic aberration correcting lens group 116and a reflecting mirror 117. Further, the illumination light illuminatesa predetermined part of the wafer 107 through the reduced projectionlens 104.

In this embodiment, the illumination light source 111 comprises anoptical fiber 118 or any other suitable light conducting means forconducting light from the exposure light source 102. A band pass filter120 is disposed on the optical path of the illumination light to permitonly E-line (λ=546 nm) and D-line (λ=589 nm) to pass therethrough out oflight wave lengths provided from the mercury lamp as the exposure lightsource 102. The illumination light which has passed through the bandpass filter 120 is incident on the half mirror 114 as a continuousspectrum light in a visible light range. Making it such a continuousspectrum light is advantageous in that the formation of interferencefringe caused by a monochromatic light is prevented to thereby enhancethe detection accuracy.

The illumination light which has been radiated onto the wafer 107 fromthe half mirror 114 via the chromatic aberration correcting lens group116, reflecting mirror 117 and reduced projection lens 104, illuminatesthe predetermined alignment pattern 108 formed on the wafer 107, thenthe reflected light travels backward through the optical path of theabove illumination light and is incident on a TV camera 121. As to theoptical path of the illumination light, it will be described in moredetail later.

The optical axis moving portion 113 has a shutter 123 formed with anillumination aperture 122 extending therethrough and a shutter support125 which is movable along a guide 124 integrally with the shutter 123.The shutter support 125 can be finely adjusted by ±l in the horizontaldirection by a stepping motor 126 as shown in FIGS. 9(a) and 9(b).

More specifically, when the shutter 123 is moved by a predetermineddistance ±l in the horizontal direction by the rotation of the steppingmotor 126, the illumination aperture 122 formed in the shutter 123 isalso moved ±l in the transverse direction in the same figures, so thatthe optical axis of the illumination light is inclined at apredetermined angle relative to the condenser lens 112.

With such inclination of the optical axis, the illumination light whichis radiated onto the alignment pattern 108 of the wafer 107 through thereduced projection lens 104, is also inclined at a predetermined angle.In this embodiment, the shutter 123 is moved ±l right- and leftwardseach by a predetermined distance with respect to a vertical referenceoptical axis, namely, an axis perpendicular to the shutter surface,whereby the illumination light can be applied to the alignment pattern108 at an angle of ±θ.

The chromatic aberration correcting lens group 116 used in thisembodiment will be described below.

Prior to the explanation of the lens group 116, principles of chromaticaberration which occurs in the reduced projection exposure techniquewill now be explained briefly with reference to FIGS. 10(a) and 10(b).In these figures, the reference marks a and b represent distances up toimaging positions from a lens 220. If the focal length of the lens 220is f, then a, b and f are in the following relationship as known well:

    1/a+1/b=1/f                                                (1)

when the focal length changes by Δf, the change Δb of the imagingposition is as follows:

    Δb=(b.sup.2 /f.sup.2) Δf                       (2)

On the other hand, as shown in FIG. 10(b), the focal length, f, andrefractive index, n of the lens 220 are in the following relationship:

    f=R/(n-1)                                                  (3)

In the above equation, R represents a radial length of the sphericalsurface of the lens 220. From the equation (3), the change Δf in focallength caused by a change Δn in refractive index is ##EQU7##Substituting the equation (4) into the equation (2) gives:

    Δb=-b.sup.2 Δn/f(n-1)                          (5)

Since the imaging magnification, m, is b/a, the equation (5) may bewritten as:

    Δb=-f(1+m).sup.2 Δn/(n-1)                      (6)

From the equation (6) it can be seen that the imaging position changesby Δb with the change Δn of the refractive index, n.

Since the refractive index, n, and the wave length of light areinversely proportional to each other, the imaging position shifts by Δbtowards the lens 220 as the wave length of light incident on the lens220 becomes longer. This is chromatic aberration.

The reduced projection lens 104 used in the reduced projection exposuresystem is generally designed to have optical characteristics mostsuitable to G-line (λ=436 nm) as the exposure light, so consideration isnot given to the problem of chromatic aberration in the use of light ofanother wave length such as E- or D-line as the illumination light.

Therefore, where a continuous spectrum light of E- and D-lines is usedas the illumination light, as previously noted, the E-line of arelatively short wave length will be focused near the lens, while theD-line of a relatively long wave length will be focused in a positionfarther from the lens. According to our calculation, the difference inimaging position in the TV camera 121 between both lines which havepassed through the optical system for alignment without being subjectedto the correction of chromatic aberration is as large as several tenmillimeters.

The chromatic aberration correcting lens group 116 in this embodiment isused for correcting the above chromatic aberration, namely, thedeviation of the imaging position in the use of a continuous spectrumlight as the illumination light.

More specifically, the lens group 116 has the function of maintainingthe imaging position constant irrespective of whether the incident wavelength is large or small. It is adjusted so that the imaging distancefrom the lens is made small with respect to light of a large incidentwave length, namely, a small refractive index, n, and made large withrespect to light of a small incident wave length, namely, a largerefractive index.

The chromatic aberration correcting lens group 116 comprises a suitablecombination of concave lenses of flint glass and convex lenses of crownglass, for example, whereby the correction of chromatic aberration canbe made in a wavelength range including E- and D-lines of Δ=500 nm to590 nm. The chromatic aberration correctable range can be easilyadjusted by changing the lens spacing in the above combined lenses.

FIGS. 11(a) and 11(b) shows in detail a sectional structure and a planarstructure, respectively, of the alignment pattern 108 formed on thewafer 107. On a semiconductor substrate 131 is formed a first pattern131a in synchronism with the formation of a wiring pattern usingaluminum (Al). Edge shapes (L, R) of the first pattern 131a arereflected in upper layers which are second and third patterns 131b,131c. The following is a more detailed explanation about the procedureof forming the patterns 131a-131c.

After the first pattern 131a was formed on the semiconductor substrate131 in synchronism with the formation of a first wiring (not shown), afirst insulating film 132a is formed through a photoresist process. Thefirst insulating film 132a is first formed to cover the whole surface ofthe alignment pattern 108, thereafter the first insulating film 132formed on the upper surface of the first pattern 131a is removed byetching again through a photoresist process, whereby the surface of thefirst pattern 131a is exposed. In this state, the second pattern 131b isformed in synchronism with a second wiring. Thus, in this embodiment,the first and second patterns 131a, 131b are laminated to each otherdirectly without intervention of the first insulation film 132a, so thedistortion of pattern caused by the interposition of an insulating filmis relatively prevented.

After formation of the second pattern 131b, a second insulating film132b is formed again by a photoresist process, then it is partiallyremoved and exposed by etching. Thereafter, a third pattern is formed onthe second pattern 131b in the same procedure as above.

Thus, according to this embodiment, since the first to third patterns131a-131c are laminated together directly without the interposition ofinsulating films, even the third pattern 131c has edges of relativelyreduced distortion in which are reflected the shapes of the underlyingfirst and second patterns 131a, 131b.

The alignment procedure using the thus-formed alignment pattern 103 (thethird pattern) will be described below. Although the alignment pattern108 in FIGS. 12 to 14 is shown in a simplified manner, its sectionalshape is about the same as that shown in FIG. 11.

In FIG. 12, the alignment pattern 108 is formed in a symmetric shape,that is, in a distortion-free state, while in FIG. 13, it is formed inan asymmetric shape due to variations in the wafer layer formingprocesses, for example, unevenness in film thickness of the first andsecond patterns 131a, 131b in FIG. 11. The alignment pattern 108 in FIG.12 is broken at its edge portions.

In the case where the alignment pattern 108 is formed in a generallysymmetric, ideal state, a high alignment accuracy can be attained by thealignment method of the prior art, that is, even with only a straightline (a vertical illumination light S). In the actual manufacturingprocess for the wafer 107, however, it is difficult to form thealignment pattern 108 always in a symmetric shape like the above; inmany cases, the alignment pattern 108 is asymmetric in section as shownin FIGS. 13 and 14.

The following description of the alignment procedure will first refer tothe case where the alignment pattern 108 is formed in a symmetric shapeas shown in FIG. 12 and then refer to the case where it is formed in anasymmetric shape as in FIGS. 13 and 14.

When an illumination light is radiated from the illumination lightsource 111 upon movement of the XY stage 106, it reaches the shutter 123through the condenser lens 112. Since the illumination aperture of theshutter 123 is moved in the direction of -l together with the shuttersupport 125 by the operation of the stepping motor 126 [FIG. 9(b)], theillumination light which has passed through the illumination aperture122 advances to the optical system for alignment which follows, in aninclined condition by -θ with respect to the vertical reference opticalaxis.

The illumination light further passes through the relay lens 115 and thechromatic aberration correcting lens group 116. At this stage theillumination light is corrected its chromatic aberration caused by acontinuous spectrum light, then it is radiated onto the alignmentpattern 108 via the reflecting mirror 117 and the reduced projectionlens 104.

At this time, the illumination light (R detection light) is applied tothe alignment pattern 108 in a displaced condition with respect to thevertical reference optical axis by the movement of the shutter 123 onthe optical path of the light. It is FIG. 12(a) that shows this state.According to the same figure, the illumination light is radiated as Rdetection light obliquely from above at an inclination angle of -θ withrespect to the vertical reference optical axis in the alignment pattern108.

The reflected light from the alignment pattern 108 which received the Rdetection light travels backward through the optical path of the Rdetection light in the manner described above and is incident on the TVcamera 121, in which it is subjected to an opto-electro conversion intoan electric signal of such a wave form as shown in FIG. 12(b). In thisfigure, the trough portions of the signal wave form correspond to theedge positions of the alignment pattern 108. By reading this coordinatesit is made possible to recognize the edge positions on the L (X_(L)) andR (X_(R)) sides of the alignment pattern 108.

As is seen from the same figure, the R detection light having suchinclination angle -θ is shaded by the left-hand edge L of the alignmentpattern 108 shown in (a) of the same figure, which shaded portioninfluences the waveform signal, making it impossible to detect an exactedge position X_(L) on the left-hand side (L). On the other hand, in thedetected edge wave form on the right-hand side (R) there is contained nonoise component such as shading of illumination caused by the edge R, soit is possible to detect the R edge position X_(R) accurately.Therefore, where the edge position detection is performed using Rdetection light, only the R edge position data X_(R) is adopted and theL edge position data X_(L) containing noise component is not used.

Next, when the shutter 123 assumes the position of +l With furtherrotation of the stepping motor 126 [FIG. 9(d)], the illumination lighttravels on its optical path at an inclination angle of +θ with respectto the vertical reference optical axis and is applied onto the alignmentpattern 108. At this time, as shown in FIG. 12(e), the illuminationlight is now an L detection light inclined by +θ with respect to thereference axis in the alignment pattern 108.

It is FIG. 12(f) that shows the wave form thereby detected. From thisfigure it is seen that the detection of R edge position X_(R) isdifficult due to noise component under the influence of shading, but thedetection of L edge position X_(L) can be done accurately and surelybecause the left-hand-side signal waveform contains no noise componentinduced by illumination shading of the edge L.

Also in this case, only L edge position data X_(L) obtained by Ldetection light is adopted and R edge position data X_(R) containingnoise component is not used.

Thus, in this embodiment, detection lights (R and L detection lights)having most suitable inclination angles (±θ) are used to detect R and Ledge positions X_(R), X_(L), respectively.

When the shutter 123 assumes the ±0 position with rotation of thestepping motor 126, that is, when the illumination aperture ispositioned on the vertical reference optical axis, the illuminationlight serves as the vertical illumination light S like the prior art. Asshown in FIGS. 12(a), (c) and (e), by the radiation of the verticalillumination light S alone, it is possible to accurately detect both Rand L edge positions (X_(L), X_(R)) as long as the alignment pattern 108is formed in an ideal state, that is, formed highly accurately in asymmetrical sectional shape.

However, where the sectional shape of the alignment pattern 108 isasymmetric, it is in many cases difficult to effect an accurate positiondetection from the detected signal wave form recognized by the TV camera121 because the reflected light contains noise component due to theinfluence of irregular reflection, etc.

When the sectional shape of the alignment pattern 108 is asymmetric, thealignment technique of this embodiment is most effective in thefollowing case.

Where the bottom of the alignment pattern 108 is inclined at an angle ofsay 2 to 5 degrees as shown in FIG. 13, the use of such a verticalillumination light S as shown in FIG. 13(c) causes noise component to beincorporated in the detected edge waveform on the right-hand side asshown in (d) of the same figure.

On this regard, this embodiment employs the R detection light describedabove, whereby the detection of R edge position (X_(R)) can be doneaccurately as shown in FIG. 13(a). With the vertical illumination lightS in the prior art, as shown in FIG. 13(c), the reflected light from theinclined bottom is incorporated in the detected edge waveform on theright-hand side, so it is difficult to effect an accurate detection of Redge position [FIG. 13(d)].

On the other hand, in the detection of L edge position, it is possibleto effect the said detection accurately by using L detection lighthaving an inclination angle of +θ, as shown in FIGS. 13(e) and 13(f),likewise, in the detection of R edge position, it is possible to effectthe said detection accurately by using R detection light having aninclination angle of -θ, as shown in (a) and (c) of the same figure.

The incorporation of noise component in the detected edge waveform iscaused not only by such inclined bottom of the alignment pattern as inFIG. 13 but also by a droop surface 133 of a corner of the steppedportion.

More specifically, when the vertical illumination light S is radiated toan alignment pattern 108 having the droop surface 133, as shown in FIG.14(c), noise component is incorporated in the detected edge waveform onthe left-hand side where the droop surface 133 is present as shown in(d) of the same figure, thus making it difficult to effect an accuratedetection of L edge position.

Even in such a case, according to this embodiment, it becomes possibleto effect an accurate detection of R edge position X_(R) by using the Rdetection light shown in FIG. 14(a), while by using L detection light itis possible to detect L edge position X_(L) accurately. Thus, by using Redge position data X_(R) obtained using R detection light and L edgeposition data X_(L) obtained using L detection light, it is possible todetect both R and L edge positions accurately without being influencedby irregular reflection from the droop surface 133.

The edge detecting operation described above will be explained belowusing the flowchart of FIG. 15(a) at the level of signal processing.

First, R edge position is detected using R detection light and thedetected position coordinates are designated X_(R) [step 251 in FIG.15(a)]. At this time, L edge position data are not adopted.

Next, L edge position is detected using L detection light and thedetected position coordinates are designated X_(L) (step 252). In thiscase, R edge position data are not adopted.

Average coordinates of X_(R) and X_(L) thus obtained are calculated andthe result is recognized as reference pattern position coordinates X_(O)(step 253).

The following description is now provided about detecting asymmetricityof the alignment pattern 108 using the R detection light, verticalillumination light S and L detection light described above.

First, X_(O) is determined by the signal processing of FIG. 15(a) (step254), then the vertical illumination light S shown in FIGS. 12(c) and13(c) is radiated to the alignment pattern 108 to obtain R detected Rand L position coordinates (Z_(R), Z_(L)), and thereafter calculation ismade to obtain a mean value, Z_(O), of the two. Of course, since eitherthe coordinates Z_(R) or Z_(L) (R or L edge position data) obtainedcontains noise component as explained above, the mean value Z_(O)obtained therefrom is not a true value.

Then, the X_(O) obtained in step 253 is subtracted from the Z_(O) tocalculate ΔZ (step 256) which represents the deviation of Z_(O) from atrue reference pattern position coordinates XO_(O). In the subsequentalignment processing, therefore, by adding ΔZ as the amount ofcorrection to the Z_(O) obtained using the vertical illumination lightS, there can be obtained a true reference pattern position coordinatesX_(O) without using R and L detection lights.

Thus, in this embodiment, once the reference pattern positioncoordinates X_(O) are obtained using R and L detection lights, then inthe subsequent alignment operations it is possible to recognize thepattern position accurately with respect to each alignment pattern 108by a mere arithmetic processing of adding the amount of correction ΔZ tothe value detected using the vertical illumination light S.

FIG. 16 is a partial system diagram showing the arrangement ofillumination light sources used in a reduced projection exposure systemaccording to embodiment-2(II) of the present invention.

In this embodiment-2(II), the optical axis moving described in the aboveembodiment-2(I) is not used, but instead two illumination light sourcesare provided to obtain the same effects as in the embodiment-2(I).

More specifically, a first illumination light source 141a is disposed sothat the light emitted therefrom is applied to the alignment pattern 108as a detection light R at an angle of +θ with respect to the verticalreference optical axis, while a second illumination light source 141b isdisposed so that the light emitted therefrom is applied to the alignmentpattern 108 as a detection light L at an angle of -θ relative to thevertical reference optical axis.

The first and second illumination light sources 141a, 141b may be of thesame construction as in the previous embodiment-2(I) wherein the lightemitted from the exposure light source 102 is conducted to theillumination light source through the optical fiber 118. Or they mayeach comprise an independent light source.

In this embodiment there can be obtained the same R and L detectionlights as in the embodiment-2(I) by changing over between the first andsecond illumination light sources 141a, 141b, thus permitting anaccurate edge position detection.

Although the present invention has been described concretely in terms ofembodiments, it goes without saying that the invention is not limitedthereto and that various modifications may be made within the range notdeparting from the gist of the invention.

For example, although in the embodiment-2(I) there was used as acomponent of the illumination light source 111 the optical fiber 118 forconducting the light of the mercury lamp as the exposure light source102 to the illumination light source, there may be used a xenon lamp orany other suitable lamp which is independent as the illumination lightsource 111. By using a xenon lamp which emits a relatively uniform lightenergy at various wave lengths, it becomes possible to adopt acontinuous spectrum light selectively with the result that it ispossible to effect a high accuracy pattern detection free ofinterference fringe.

Although in the above description the present invention was applied tothe alignment technique in the so-called reduced projection exposure asa utilization field of the invention, this constitutes no limitation andthe invention is applicable also to the photo mask--wafer alignmenttechnique in a 1:1 magnification exposure for example.

Typical effects attained by the above embodiments-2(I) and -2(II) willbe described below briefly.

By radiating the illumination light at a predetermined inclination anglerelative to the vertical reference optical axis for a pattern, itbecomes possible to accurately detect the position of one of both edgesof the pattern. Therefore, by repeating radiation of the illuminationlight to the pattern while varying such inclination angle, it becomespossible to effect an accurate position detection even in the case of adistorted pattern, namely, a pattern asymmetric in section, thuspermitting enhancement of the alignment accuracy.

Further, the first coordinate value obtained by the radiation ofdetection light in the inclined direction and the second coordinatevalue obtained using the vertical illumination light are compared witheach other to calculate the amount of correction, whereby the amount ofdeviation in the detection using the vertical illumination light becomesclear. Therefore, in the subsequent position detecting operations, it ispossible to conduct an efficient alignment processing by correcting dataobtained using only the vertical illumination light.

(3) Embodiment-3

FIG. 18 is a perspective view showing principal components of a reducedprojection exposure system having an alignment device according toembodiment-3 of the present invention; FIGS. 19 and 20 are explanatoryviews showing a light source portion used therein.

The reduced projection exposure system of this embodiment is providedwith an optical exposure system comprising an exposure light source 301which comprises a mercury lamp for example, a condenser lens 302 forcondensing the exposure light emitted from the exposure light source301, and a reduced projection lens 303.

As to the mercury lamp and the reduced stepping projection aligner,reference is here made, in place of description herein, to Ichiro Hoko,"Semiconductor Lithography Technique," the second edition, Sankyo ToshoK.K. (May 30, 1986), pp. 81-102.

Between the condenser lens 302 and the reduced projection lens 303 isremovably disposed a master plate 304 such as a reticle fabricated bycoating a transparent glass substrate or the like with a light shieldingfilm of a desired integrated circuit pattern enlarged to a predeterminedmagnification (e.g. 5×).

Under the reduced projection lens 303 is located a semiconductor wafer305 (an object to be exposed) which is placed on an X-Y stage movably ina horizontal plane and which has been coated on the surface thereof witha photoresist film 305a, for example, by a rotational applicationtechnique in the preceding application process.

The exposure light emitted from the exposure light source 301 and passedthrough the master plate 304 is reduced to a predetermined magnification(e.g. 1/5) by the reduced projection lens 303 and then projected ontothe semiconductor wafer 305, whereby the photoresist film 305a formed onthe surface of the wafer is exposed to the integrated circuit pattern ofthe actual size.

Near the above optical exposure system is disposed an illumination lightsource 306, which emits an illumination light 307. The illuminationlight 307 passes through a condenser lens 308 and is reflected at apredetermined angle by a half mirror 309, then reaches the reducedprojection lens 303 via a relay lens 310, a chromatic aberrationcorrecting lens group 316 and a reflecting mirror 311; further, throughthe reduced projection lens 303 the illumination light illuminates apredetermined part of the wafer 305.

In this embodiment, the illumination light source 306 includes a lightconducting means such as an optical fiber 306a for conducting light fromthe exposure light source lamp. On the optical path of the illuminationlight is disposed a band pass filter 317 to permit only E-line (Δ=546nm) and D-line (λ=589 nm) to pass therethrough out of the wave lengthsof light emitted from the mercury lamp as the exposure light sourcelamp. The illumination light which has passed through the band passfilter 317 is incident on the half mirror 309 as a continuous spectrumlight in a visible light range. The use of such a continuous spectrumlight is advantageous in preventing an interference fringe caused by theuse of a monochromatic light, thereby enhancing the detection accuracy.

The illumination light 307 radiated onto the wafer 305 via the halfmirror 309, chromatic aberration correcting lens group 316, reflectingmirror 311 and reduced projection lens 303 illuminates a predeterminedalignment pattern formed on the wafer, then as the reflected light ittravels backward through the optical path thereof and is received by aTV camera 312.

The TV camera 312 is connected to a signal processor through a signalcable 312a, and the position of an alignment mark 305b is determined bycalculation on the basis of the wave form of a detected signal providedfrom the TV camera 312 and reflecting the shape of the alignment mark305b.

The illumination light source 306, as shown in FIG. 19, comprises theoptical fiber 306a (a multi-spot light source) for conducting a desiredwave length light as the illumination light 307 out of a light sourcesuch as a mercury lamp, and a cylindrical mirror 306b (reflecting means)which is coaxially mounted on an end portion of the optical fiber 306aand whose inner peripheral surface is a mirror surface 306c formed by acylindrical or conical surface. The illumination light 307 radiated anddiffused from the end portion of the optical fiber 306a is reflected tothe optical axis side by the inner peripheral surface of the cylindricalmirror 306b and then radiated to the exterior.

As to the cylindrical mirror used herein, reference is here made, inplace of description herein, to Shiba et al.'s Japanese Patent Laid-OpenNo. 251858/86 (laid open Nov. 8, 1986, filed May 1, 1985 as ApplicationNo. 92302/85) in which is shown an example of application of thecylindrical mirror to an illumination system in an optical exposuresystem.

In this embodiment, as shown in FIG. 21, the illumination light 307radiated from the optical fiber 306a is reflected by the innerperipheral surface of the cylindrical mirror 306b, whereby it becomes aring-shaped light beam in the position of an entrance pupil 303a of thereduced projection lens 303. And there is performed a non-Koler'sillumination wherein a desired area of the semiconductor wafer 305 isirradiated in a spot form with an incoherent illumination light 7comprising unparallel light rays.

The operation of this embodiment will be described below.

First, the X-Y stage is moved to make adjustment so that theillumination light 307 radiated from the optical fiber 306a of theillumination light source 306 and applied to the semiconductor wafer 305via the cylindrical mirror 306b, condenser lens 308, beam splitter 309,relay lens 310, reflecting mirror 311 and reduced projection lens 303,is directed to the alignment mark 305b formed on a predetermined part ofthe wafer 305.

At this time, according to this embodiment, a sectional image of theoptical fiber 306a of the illumination light source 306 and a "virtualimage" of the illumination light source 306 comprising a ring-shapedlight beam which surrounds the said sectional image, are formed in theposition of the entrance pupil 303a of the reduced projection lens 303,as shown in FIG. 22, and the illumination light 307 which illuminatesthe alignment mark 305b on the surface of the semiconductor wafer 305becomes an unparallel light beam and incoherent.

Consequently, the intensity of interference fringe of the illuminationlight 307 in the transparent photoresist film 305a formed on the surfaceof the wafer 305 becomes smaller, and the illumination light isreflected from the surface of the alignment mark 305b formed unevenly onthe wafer surface, so that the reflected light, indicated at 307a, whichreflects the shape of the alignment mark 305b is applied to the TVcamera 312.

As a result, there is obtained a detection signal which clearly reflectsthe shape of the alignment mark 305b, like a detection signal Aindicated by a solid line in the diagram of FIG. 23. For example, bydetecting minimal points of the detection signal A corresponding to thestepped portions of the alignment mark 305b and calculating a middlepoint thereof, it is possible to grasp the position of the alignmentmark 305b accurately.

Consequently, the alignment of a desired part of the semiconductor wafer305 relative to the master plate 304 can be done with a high accuracy onthe basis of the position of the alignment mark 305b, thus permittingimprovement of the alignment accuracy.

On the other hand, in the conventional Koler's illumination wherein theillumination light 307 is radiated as parallel light rays to thesemiconductor wafer 305, there occurs a strong interference fringe inthe photoresist film 305a, so that a detection signal B of theinterference fringe is incorporated in the detection signal A whichreflects the shape of the alignment mark 305b, and there is detected adetection signal C having an indistinct wave form not reflecting theshape of the alignment mark. Consequently, it becomes difficult toaccurately grasp the position of the alignment mark 305b and thedeterioration in alignment accuracy of the semiconductor wafer 305relative to the master plate 304 is unavoidable.

And on the basis of the position of the alignment mark 305b which hasbeen grasped accurately, a desired area of the semiconductor wafer 305is positioned accurately on the optical axis of the optical exposuresystem, and thereafter the integrated circuit pattern formed on themaster plate 304 is transferred onto the photoresist film 305 of thewafer 305 by the exposure light emitted from the exposure light sourceand passed through the condenser lens 302, master plate 304 and reducedprojection lens 303.

(4) Explanation of Light Sources Employable in the Embodiments

FIGS. 27(a) and 27(b) are each a schematic wave length distributiondiagram of a light source for reference suitable for the embodiments ofthe present invention, (a) using a dichromatic light (E- and D-lines)from a mercury lamp and (b) using a continuous light or "white light",e.g. a visible light ranging from green to orange color, (a 100 nm widecontinuous spectrum of mainly 560 nm) from a xenon lamp. These can besuitably selected as necessary.

On the other hand, as to the exposure light, explanation is made aboutG-line (λ=436 nm) of a mercury lamp which is a monochromatic light, butas other light sources there are employable mercury lamp's h-line (405nm) and i-line (365 nm), as well as XeCl line of 308 nm , KrF line of248 nm, and ArF line of 193 nm using Excimer laser.

(5) Explanation of Alignment Process and Exposure/Manufacturing Processcommon to the Embodiments

Although the following explanation is concerned with embodiment-3 as anexample, it is applicable as such to the other embodiments.

A concrete semiconductor integrated circuit manufacturing process aswell as the structure and operation of the exposure system, using thetechnique of the present invention, will be described below.

FIG. 28 shows an interrelation between a projection lens barrel 303(comprising ten-odd lenses) and a pair of optical barrels 326a, 326b forglobal alignment or pre-alignment.

FIG. 29 shows a path of a main light ray in an optical exposure systemof the reduced stepping projection exposure system illustrated in FIG.18. From this figure it can be seen that the wafer side, i.e., the imageside, is telecentric.

FIG. 35 shows positions of pre-alignment marks (for rough alignment andθ azimuthal alignment) on a wafer to be processed.

FIG. 36 shows the arrangement of marks for chip alignment or finealignment.

FIG. 37 shows the arrangement of scribe lines, i.e., street (Xdirection) and avenue (Y direction), on the device surface on the wafer.

FIG. 38 shows the shape of alignment marks on the scribe lines for usein fine alignment and pre-alignment.

In FIGS. 28, 29, 35 and 38, the lens barrel 303, which is for a reducedprojection exposure of 10:1 or 5:1, comprises ten-odd lenses (formonochromatic light of g- or i line) and can expose an area on the waferof about 20 mm×20 mm to light in one shot. The optical barrels 326a and326b are for global alignment or for pre-alignment (rough alignment andθ azimuthal alignment) such as die-by-die or site by site alignment, andthey are disposed in positions not coaxial with the projection barrel.The pre-alignment is performed using light which has been renderedresist-insensitive by removing an ultraviolet region using a filter froma visible white light source such as a halogen lamp.

Numeral 301 denotes a mercury lamp as a light source; numeral 302denotes a condenser lens; numeral 303 denotes a projection lens group (aprojection lens barrel); numeral 303a denotes an entrance pupil plane;and numeral 303b denotes an example of a projection lens. Numeral 304denotes a reticle or a mask; numeral 305 denotes a wafer; numeral 305adenotes a resist film; and numeral 327 denotes an X-Y stage for steppingexposure which is controlled by a laser interferometer. Numeral 328represents a main light ray passing through the center of the entrancepupil.

Numerals 341a and 341b denote pre-alignment marks on the device surfaceof the semiconductor wafer; and numerals 342a to 342j denote like marksfor chip alignment (fine alignment). All of these marks are of the sameshape and their details are illustrated in FIG. 38. These marks aredisposed, one for the area of one shot, on the scribe lines (343:street, 344: avenue) throughout the wafer and they are suitably usedselectively.

In FIG. 38, the scribe line (avenue) indicated at 344 is about 150 μmwide; numerals 345a and 345b each denote a chip area; numerals 346a and346b each denote an alignment pattern element in the X direction, 2 μmin width and 50 μm in length; numerals 347a and 347b each denote analignment pattern element in the Y direction, having the same size asthat of the X-pattern elements. Numerals 348a to 348b represent obliquepattern elements.

In FIGS. 30 to 34, numeral 305 denotes a wafer; numeral 330 denotes aLOCOS oxide film; numeral 331 denotes a gate oxide film; numeral 332denotes a PSG film (first passivation or PSG.I); numeral 333 denotes afirst-layer aluminum wiring (Al.I); numeral 333a denotes an aluminumwiring of the same layer as the Al wiring 333, which defines an opening350 for alignment mark. The alignment mark opening 350 corresponds, forexample, to the pattern element 347a shown in FIG. 38. Numeral 334denotes an inter-layer PSG film (PSG.II) and numeral 335 denotes apositive type photoresist film formed in a thickness of about 1 μmrepresents using a spinner. Numeral 336 represents an opening of theresist film for opening a through hole in the PSG II and also representsthe said through hole, and numeral 337 denotes a second-layer aluminumwiring (Al.II).

An explanation of the exposure process will now be given with respect toan example of forming a through hole and a contact hole between thealuminum layers in the two-layer aluminum memory process. The details ofthe global alignment or pre-alignment method are disclosed in Nakazawaet al's Japanese Patent Laid-Open No. 102823/81 (laid open Aug. 17,1981), and as to DRAMS's two layer Al process, it is explained in Murataet al's Japanese Patent Application No. 235906/87 (filed Sept. 19,1987), so reference is here made thereto in place of describing theirdetails herein.

First, a wafer having such a resist film as in FIG. 30 formed throughoutthe entire surface thereof is placed and attracted onto the X-Y stage327 so that the orientation flat of the wafer assumes an approximatelycertain configuration, as shown in FIG. 29. Next, the optical barrels326a and 326b for global alignment shown in FIG. 28 are brought intoalignment with the paired alignment marks 341a and 341b as in FIG. 35,whereby the θ azimuth and the XY position are primary aligned. This willhereinafter be referred to as the "off-axis prealignment". (He--Ne laserbeam of 543.5 nm is used.)

Subsequently, the XY table is moved to detect the alignment marks342a-342j (eight to ten marks are selected from among a larger number ofalignment marks) shown in FIG. 36 successively using E-line according tothe TTL method, as illustrated in FIG. 18. Each detection is effected byseeing the alignment pattern elements 346a (X direction) and 347a (Ydirection) shown in FIG. 38 in a bright field. And stepping exposure isperformed successively under movement of the stage so that the entirechip alignment deviation becomes optimal on the basis of the multi-spotdeviations detected. Exposure is effected in such a manner as shown inFIG. 29.

As a result of the exposure, only the resist film area corresponding tothe contact hole portion becomes easier to be removed, permitting anopening 336 to be formed. Using this opening, the PSG.II, i.e., 334, isetched selectively, whereby a through hole is formed in a predeterminedposition of the PSG.II as shown in FIG. 32. Sebsequently, thephotoresist film 335 is removed throughout the entire surface thereofand the second-layer aluminum film 337 is formed by sputtering on thewhole surface of the PSG.II.

The structure of the illumination light source 306 is not limited to theone shown in FIG. 19. For example, there may be adopted a structurewherein a plurality of condenser lenses 306d, 306e are attached to bothend portions of the cylindrical mirror 306b. Also in this case, aring-shaped light beam is focused on the entrance pupil of the reducedprojection lens 303 and the illumiantion for the semiconductor wafer 305by the illumination light 307 becomes an incoherent non-Kolerillumination, thus permitting the same effects as in the use of thecylindrical mirror 306b to be obtained.

Thus, the following effects can be obtained in this embodiment.

(1) The illumination light source 306 comprises a multi-spot lightsource such as the optical fiber 306a and the cylindrical mirror 306bhaving an inner peripheral surface formed by the mirror surface 306c anddisposed about the optical axis of the illumination light 307 which isemitted from the optical fiber 306a. The illumunation light 307 which isradiated after being reflected by the cylindrical mirror 306b becomes aring-shaped light beam in the position of the entrance pupil of thereduced projection lens 303 and illuminates the surface of thesemiconductor wafer 305 as non-Koler illumination of low coherence.Therefore, without formation of a strong interference fringe by thetransparent photoresist film 305a formed on the surface of the wafer305, it is possible to obtain a detection signal A which clearlyreflects the shape of the alignment mark 305b on the basis of thereflected light 307a from the same alignment mark, and thus it becomespossible to grasp the position of the alignment mark 305b accurately.

Consequently, the alignment of a desired part of the semiconductor wafer305 relative to the master plate 304 based on the position of thealignment mark 305b can be effected with a high accuracy, thuspermitting improvement of the alignment accuracy.

(2) As a result of the above effect (1), the integrated circuit patternsformed laminatedly on the semiconductor wafer 305 can be improved intheir overlapping accuracy, thus permitting improvement in yield of thesemiconductor device.

(3) As a result of the above effects (1) and (2), the productivity inthe wafer processing step in the production of the semiconductor devicecan be improved.

It goes without saying that the present invention is not limited to thisembodiment and that various modifications may be made within the rangenot departing from the gist of the invention.

For example, the reflecting means is not limited to a cylindricalmirror; it may be of any other shape, e.g. prismatic or pyramidal shape.

Further, the optical fiber of the illumination light source may beconnected to the exposure light source to utilize part of the exposurelight as the illumiantion light.

Although in the above description the present invention was applied tothe alignment technique in the reduced projection exposure which is abackground utilization field of the invention, the invention is notlimited thereto but is widely applicable to the alignment technique ingeneral reduced projection exposure.

According to this embodiment, the alignment apparatus in reducedprojection exposure is provided with a light source and a reflectingmeans for reflecting illumination light emitted from the light sourcetoward the optical axis of the illumination light, the reflecting meansbeing disposed around the said optical axis, in which an alignment markformed unevenly on an object to be exposed is detected its position bynon-Koler illumination involving radiation of the said illuminationlight comprising unparallel light rays to the said alignment mark.Consequently, the illumination light applied to the alignment markformed on the object to be exposed becomes incoherent, so that, forexample, in a transparent photoresist layer formed in an irregularthickness on the surface of the object to be exposed, the formation of acomplicated and strong interference fringe which does not reflect theshape of the alignment mark is avoided.

As a result, a signal of the irregular interference fringe will notincorporate in the detection signal based on the intensity of thereflected light which reflects the shape of the alignment mark, so it ispossible to detect the position of the alignment mark clearly and theaccuracy in the alignment of the object to be exposed based on thealignment mark relative to the master plate can be improved.

(6) Details of Chromatic Aberration Correcting System and Correction ofHigher Order for Chroamtic Aberration

The following description is mainly about the case where there is usedsuch a "white light" (continuous spectrum) as in FIG. 27(b) in anoptical alignment system using a monochromatic g-line exposure (NA=0.38of the projection lens group 3) from a mercury lamp as in embodiment-1.But it is also applicable as such to the other light sources andembodiments, so the explanation of the others will be omitted.

Further, the exposure light is not limited to g-line; there also may beused h- and i-lines, as well as Excimer laser light.

The stepper used herein is based on the 5:1 Stepper 6400 DSW of GCA Co.(U.S.A.), and the projection lens 3 is a g-line projection lens Model10-78-46 (NA 0.38) of Zeiss Co. (West Germany). In general, a lenssystem of this type comprises several to several ten lenses for removingvarious aberrations.

FIG. 39 is a sectional view in the Y direction (vertical direction)showing an entire layout of a chroamtic aberration and astigmatismcorrecting lens system in the optical alignment system. In the samefigure, the numeral 618 denotes an end mirror for taking out analignment light to the outside of the axis of the projection lens 3 in aposition below the mask or reticle 4; numeral 602 denotes an alignmentlight beam which is fed to the projection lens system 3 according tothrough-the-lens method; numeral 605 denotes an imaging point; numerals603 and 607 each represent an achromatic lens group disposedsubstantially in a relation of point symmetry with respect to theimaging point; numerals 604 and 606 represent a pair of cylindricallenses (crown glass lenses) for the correction of astigmatism, whichlenses are also disposed substantially in a relation of point symmetrywith respect to the imaging point 605. Numeral 615 denotes an achromaticrelay lens (focal length: 36 mm) and numeral 608 denotes an alignmentlight beam travelling toward the beam splitter (FIG. 1).

FIG. 40 is a sectional view in the X direction (horizontal direction) ofthe above correction lens group. In the same figure there are shown inmillimeter the thickness of each lens, spacing between adjacent lenssurfaces, and radius of curvature of a spherical lens surface, (alsoshown in the following figures). The lens thickness means the distancebetween the centers (i.e., the point intersecting an optical axis 609 orZ axis) of both end faces of each laminated lens, and the spacingbetween adjacent lens surfaces means the distance between the centers ofadjacent surfaces of adjacent lenses. The reference marks A and Crepresent types of the laminated achromatic lenses.

FIG. 41 is a sectional view in the Y direction of the correction lensgroup and FIG. 42 shows the details of an achromatic lens of type A. InFIG. 42, numerals 621 and 622 denote a crown glass lens and a flintglass lens, respectively. FIG. 43 shows the details of an achromaticlens of type C, wherein numerals 623 and 624 denote a flint glass lensand a crown glass lens, respectively. If the glass materials are shownin terms of Model numbers of Schott Co., 604, 606, 621 and 624 are crownglass (BK7), while 622 and 623 are flint glass (SF11).

In such layout, most of chromatic aberrations for D- and E-lines of themercury lamp are eliminated, but without fine adjustment, there willremain to some extent a lateral deviation in the XY plane. This lateraldeviation can be removed by shifting the chromatic aberration correctinglens groups 603 and 607 oppositely in the XY plane in the direction ofthe remaining chromatic deviation while substantially ensuring thesymmetry with respect to the imaging point 605, and then fixing them ata point where the chromatic deivation disappears completely.

FIGS. 44 to 46 shows the details of fine adjustment in the correctionlens system.

In FIG. 44, numeral 609R denotes an optical axis of a reference opticalsystem which coincides with 609 in the previous figures; numeral 605represent the first imaging point as noted previously; numerals 604 and606 represent a pair of astigmatism correction lenses.

In FIGS. 45 and 46, numerals 603 and 607 each denote a chromaticaberration correcting lens group; and numerals 609A and 609B representrotation symmetry axes of the chromatic aberration correcting lensgroups 603 and 607, respectively.

Adjustment of the reference optical system is made in the followingmanner. First, the correction lens groups 603, 604, 606 and 607 areremoved from the reference optical system shown in FIG. 39, leaving onlyachromat relay lens 615, and using only E-line, the position of theentrance pupil of the projection lens 3 (FIG. 1) is confirmed.

Next, the end mirror 618 is adjusted so that the image at the terminalend (an effective light sofurce point) of the optical fiber 13 (FIG. 1)is positioned centrally of the entrance pupil.

Next, the astigmatism correction lenses 604 and 606 are positionedsymmetrically with respect to the imaging point 605 and ←Z_(A) isadjusted as in FIG. 44 to minimize astigmatism.

Then, using a mixed light of E- and D-lines, ΔZ, ΔY and ΔX are adjustedsuccessively as in FIGS. 45 and 46 in the state of FIG. 39 to minimize adeviation of focused position caused chromatically and an imagingdeviation in the image height direction while looking at the pattern onthe wafer. By so doing, with respect to E- and D-lines of the mercurylamp, the focal chromatic aberration and magnification chromaticaberration are removed in the sense of the so-called "achromat".Spherical aberration is removed by suitably designing the curvature ofeach lens surface in the correction lens groups. Other aberrations arereduced to values causing no problem by disposing the correction lensgroups symmetrically with respect to the imaging point 605 or bysuitably designing correction lenses.

(7) Explanation of Literatures, etc. for Supplementing Embodiments

The following matters are described in the literatures, etc. to bementioned, so reference will be made thereto in place of describingtheir details herein.

As to the device structure such as DRAM's wafer process and double-layeraluminum wiring, it is described in Murata et al's Japanese PatentApplication No. 235906 (filed Sept. 19, 1987), its corresponding U.S.patent application Ser. No. 246514 (filed Sept. 19, 1988) and its KoreanPatent Application No. 11906/88.

As to a reduced magnification barometric compensation system, it isdescribed in Komoriya et al's Japanese Patent Laid-Open No. 262421/85(Application No. 118315/84, filed June 11, 1984) and its correspondingU.S. Pat. No. 4,699,505 (registered Oct. 13, 1987), Shimizu et al's U.S.Pat. No. 4,606,273 (registered May 19, 1987) and Tanimoto et al's U.S.Pat. No. 4,690,528 (registered Sept. 1, 1987).

As the exposure light source, photoresist (positive type), alignment,and reduced projection exposure at large, these are described in IchiroHoko, "Semiconductor Lithography Technique," Sangyo Tosho K.K. (May 30,1986), pp. 20-26 and 81-102.

As to the cylindrical inner-surface mirror used for illumination(alignment), it is disclosed in Shiba et al's Japanese Patent Laid-OpenNo. 251858/86 (laid open Nov. 8, 1986).

As to the stepper alignment technique, it is explained in Nakazawa etal's Japanese Patent Laid-Open No. 102823/81 (laid open Aug. 17, 1981).

As to Koehler illumination, aperture or entrance pupil, aberrationcorrection, filter, and geometric optics at large, these are explainedin Hiroshi Kubota, "Optics," Iwanami Shoten (Oct. 30, 1968), pp. 41-276;ibid., "Wave Optics," ibid. (1971), pp. 199-236; F. A. Jenkins & H. E.White, "Fundamentals of Optics," McGraw-Hill Book Inc. Chapters 1-10; M.Born & E. Wolf, "Principles of Optics," Pergamon Press, pp. 133-255,418-428 and 522-526; and M. V. Klein, "Optics," John Wiley & Sons, Inc.,pp. 106-118.

Further, as to optical materials, including lenses, they are shown inKubota et al, "Optical Art handbook," Asakura Shoten (Mar. 1, 1987), pp.551-674.

(8) Explanation of Various Effects obtained by the Embodiments of theInvention

According to the present invention, in exposing a semiconductorintegrated circuit wafer to a monochromatic light such as g-, i- orh-line of a mercury lamp, using a reduced stepping exposure system, thealignment between the wafer and a mask or a reticle is performed byconducting light from a predetermined pattern on the wafer to anoff-axis position by through-the-lens (TTL) method and observing it. Inthis case, the observation light taken out from below the reticle ispassed through a chromatic aberration correcting lens, therebypermitting the use of a polychromatic or continuous spectrum light.

Moreover, since the reference light is taken out off axis before beingreflected by or passing through the mask or reticle, there is no fear ofbeing troubled by various aberrations caused by the flat plate of themask.

Further, since the reference light is taken out below the reticle, etc.,there is a sufficient space for the insertion of the optical correctionsystem, thus permitting symmetrical arrangement so as to thoroughlyremove spherical aberration, astigmatism, coma aberration, curvature ofimage field and distortion aberration, in addition to chromaticaberration.

Moreover, since the optical projection system extending from the reticleto the wafer lies on a straight line spatially, it is possible to removevarious aberrations (of the projection system) thoroughly over a wideexposure area.

Additionally, since the main optical axis of the projection system andthe gravitational axis are approximately coincident with each other,there is attained a structure difficult to be influenced by environmentand it becomes easy to control time-series changes.

Although the present invention has been described above about the casewhere the exposure light is monochromatic and the reference opticalsystem is in a bright field, the invention is not limited thereto; itgoes without saying that the invention is applicable to the case whereexposure is performed using a polychromatic or continuous spectrum lightof a wave length shorter than that of the reference light and alsoapplicable to a reference optical system in a dark field (diffraction orinterference). Further, although the present invention has beendescribed above with respect to the projection system wherein only theimage side (wafer, etc.) is telecentric, it is also applicable to asystem wherein an article side (reticle, etc.) is also telecentric.

What is claimed is:
 1. A projection exposure system wherein a pattern ona main surface of a reticle or a mask is imaged for reduced projectionexposure onto a thin photosensitive film in a step-and-repeat manner,said thin photosensitive film being formed on a first main surface of asemiconductor wafer, an integrated circuit wafer, or a plate materialwhich is used for the production of a semiconductor device of anintegrated circuit device, said projection exposure systemcomprising:(a) exposure light source means for directing a monochromaticexposure light beam of a short wave length to the main surface side ofsaid reticle or mask; (b) reticle or mask holding means for holding saidreticle or mask so that the main surface thereof is located opposedly inparallel with the first main surface of said wafer or plate material;(c) a projection lens group substantially telecentric on the image sideand comprising a plurality of lenses free of various aberrations forsaid exposure light, for imaging the pattern on the main surface of saidreticle or mask onto said photosensitive film on the first main surfaceof said wafer or plate material on a reduced scale using said exposurelight through said projection lens group; (d) wafer or plate materialholding means for holding said wafer or plate material so that the firstmain surface thereof is opposed to the main surface of said reticle ormask in a substantially parallel relation thereto, said wafer or platematerial holding means allowing said wafer or plate material to moverelatively in a plane perpendicular to a single linear optical axis ofsaid projection lens group to effect step-and-repeat exposure in asubstantially aligned condition of said optical axis of the projectionlens group, the center of a portion to be exposed of said wafer or platematerial, and the center of a portion to be projected of said reticle ormask; (e) an off-axis through-the-lens alignment optical system forobservation, said alignment optical system performing a horizontalalignment of said wafer or plate material by using reflecting means,disposed off the optical axis of said projection lens group, to direct areference light comprising one of a polychromatic light and a continuousspectrum light having a predetermined spectral width, having a wavelength longer than that of said exposure light, to the first mainsurface of said wafer or plate material through said projection lensgroup from a position off the optical axis of said projection lensgroup, and to direct reference light reflected from the first mainsurface of said wafer or plate material to the position off the opticalaxis along substantially the same path along which the reference lightis directed to the first main surface of said wafer or plate material,and by observing a predetermined alignment pattern formed on the firstmain surface of the wafer or plate material; and (f) a correction lensgroup disposed off the optical axis of said projection lens group and inan optical path of said alignment optical system for correctinglongitudinal and lateral chromatic aberrations, in the sense of anachromat, with respect to the reference light, of said projection lensgroup for the reference light reflected from the first main surface ofsaid wafer or plate material and directed to the position off theoptical axis by said reflecting means; wherein said correction lensgroup comprises a pair of substantially the same chromatic aberrationcorrecting lens group disposed substantially symmetrically with respectto a first imaging point in said alignment optical system.
 2. Anexposure system according to claim 1, wherein the angle of illuminationof an illumination light beam as a whole in said alignment opticalsystem with respect to the first main surface of said wafer or platematerial can be varied to a desired angle smaller than 90 degrees.
 3. Anexposure system according to claim 1, wherein said reflecting means isdisposed between said reticle or mask and said projection lens group. 4.An exposure system according to claim 3, wherein said correction lensgroup includes a pair of astigmatism correction lenses for thecorrection of astigmatism.
 5. An exposure system according to claim 4,wherein said paired astigmatism correction lenses are disposed insubstantially symmetric positions with respect to said first imagingpoint.
 6. An exposure system according to claim 5, wherein saidreference light comprises a continuous spectrum having a predeterminedwidth.
 7. An exposure system according to claim 6, wherein saidcontinuous spectrum is included in a visible light range.
 8. An exposuresystem according to claim 7, wherein the source of said reference lightis a halogen lamp.
 9. An exposure system according to claim 5, whereinsaid reference light is a light comprising a plurality of wave lengths.10. An exposure system according to claim 9, wherein said referencelight is included in a visible light range.
 11. An exposure systemaccording to claim 10, wherein the source of said reference light is amercury lamp.
 12. An exposure system according to claim 11, wherein thecoaxial downward illumination system using said illumination light is aKoehler illumination system which forms an image of an effective lightsource on the entrance pupil of said projection lens group.
 13. Anexposure system according to claim 12, wherein in order for part of saidreference light to be imaged on a wider portion on the entrance pupil, acylindrical inner-surface mirror is disposed in an optical vicinity ofsaid effective light source so that the optical axis of the referencelight at said portion and a rotation symmetry axis thereof becomecoincident with each other.
 14. A method of making a semiconductorintegrated circuit device having two or more aluminum wiring layers,comprising the step of using an exposure system according to claim 1 toform the aluminum wiring layers of the semiconductor integrated circuitdevice.
 15. A pattern detecting method for optically detecting a steppedpattern formed on a semiconductor element by radiating an illuminationlight to a stepped pattern portion inclinedly relative to a verticalreference optical axis through the semiconductor element, characterizedin that a first coordinate value obtained by radiating the illuminationlight to the stepped pattern portion inclinedly relative to the verticalreference optical axis through the semiconductor element and a secondcoordinate value obtained by the radiation of a vertical illuminationlight parallel to the vertical reference optical axis are compared witheach other to calculate the amount of correction for the secondcoordinate value obtained by the radiation of the vertical illuminationlight, and in a subsequent pattern detection said amount of correctionis added to a coordinate value obtained by the radiation of the verticalillumination light to calculate a true coordinate value.
 16. A patterndetecting method for detecting a pattern formed on a semiconductorelement, characterized in that in laminating a plurality ofcharacteristic layers on the surface of a semiconductor wafer, there isformed a stepped pattern having first and second edges at both endsthereof with a vertical axis through the wafer as the center, and inradiating an illumination light to said pattern and recognizingcoordinate positions of both said edges on the basis of the lightreflected from the pattern, an illumination light for the first edge andthat for the second edge are radiated through separate optical paths.17. A method of making a semiconductor device, characterized in that inlaminating a plurality of characteristic layers successively on thesurface of a semiconductor wafer using a mask, there is formed a steppedpattern of an asymmetric section having first and second edges at bothends thereof with a vertical axis through the wafer as the center, and afirst illumination light which is radiated inclinedly from the secondedge side is used in detecting the position of the first edge, while asecond illumination light which is radiated inclinedly from the firstedge side is used in detecting the position of the second edge, thencentral coordinates of both edges are calculated using coordinates ofthe first edge position obtained by the radiation of the firstillumination light and coordinates of the second edge position obtainedby the radiation of the second illumiantion light, and thecharacteristic layer formed thereon is positioned with respect to themask on the basis of said central coordinates calculated.
 18. A methodof making a semiconductor device according to claim 17, wherein thesemiconductor device includes a memory cell having two or morecharacteristic layers which are aluminum wiring layers.